1. Field of the Invention
This invention relates to the field of material forming, particularly to material forming under superplastic conditions.
2. Description of the Prior Art
Under certain conditions, some materials can be plastically deformed without rupture well beyond their normal limits, a property called superplasticity. The usual process involves placing a sheet of material in a die, heating the material to a temperature at which it exhibits superplasticity, and then using a gas to apply pressure to one side of the sheet. Sufficient pressure is applied to strain the material at a strain rate which is within the superplasticity range of the material being formed at the selected temperature. This gas pressure creates a tensile stress in the plane of the sheet which stretches the sheet and causes it to form into the die cavity. This process is described in U.S. Pat. No. 4,181,000 to C. Howard Hamilton (of the present invention), Neil E. Paton, and John M. Curnow. The process is being used increasingly for forming different configurations such as titanium sheet metal structures for aircraft.
One undesirable characteristic of many superplastic alloys is their tendency to cavitate (form small internal voids) during the tensile deformation imposed by prior art forming operations. The voids limit the superplastic ductility of the material, as well as reduce its mechanical properties if they are present in a sufficiently large volume fraction. Unfortunately, the rate of cavitation is usually maximum when superplasticity is maximum, a problem which has limited the application of prior art superplastic forming methods.